This invention relates to direct metal bonding.
Directly bonding copper (DBC) to a ceramic insulator has been done for years. Generally, a layer of copper is laid over a layer of ceramic, typically aluminum oxide (Al.sub.2 O.sub.3) or aluminum nitride (AlN), and then brought to a temperature of about 1064.degree. C. The copper layer may be oxidized before being laid on the ceramic layer, or oxygen may be injected into the furnace to cause oxidation. Alternatively, a layer of bonding material can be inserted between the copper layer and the ceramic layer. In either case, the result is a strong oxide or eutectic bond between the copper and the ceramic layers. Typically, the bonded copper/ceramic sheet is then passed through a photo-imaging process to create a conductive circuit pattern for use with a printed circuit board (PCB), but there are also other uses for the copper/ceramic sheet.
The conductive circuit pattern can be used in integrated circuit components such as power converters, referred to as integrated power devices (IPD). Military specification 883D 1010.7 requires integrated circuit components to be tested over a temperature range of -55.degree. C. to 150.degree. C. Copper has a thermal expansion coefficient of approximately 16.times.10.sup.-6 inches per inch per degree C, whereas the ceramic (aluminum oxide, Al.sub.2 O.sub.3) generally has a much lower thermal expansion coefficient of about 8.times.10.sup.-6 inches per inch per degree C. As a result, when the copper/ceramic sheet is exposed to different temperatures, the copper layer expands and contracts more than the ceramic layer.
The eutectic bond is strong and limits the expansion and contraction of the copper layer in the area of the bond. As a result, the confined copper layer exerts stress on the ceramic layer through the bond. Typically, a relatively thin layer of copper, about 0.001 inch, is able to flex and stretch without causing damage to the ceramic layer. Increasing the thickness of the copper layer, however, increases the amount of stress inflicted on the ceramic layer.
Referring to FIG. 1, if the copper/ceramic layer 10 is exposed to a high temperature, for example, 150.degree. C., and then to a low temperature, for example, -55.degree. C., the copper layer 12 will contract in a direction 14. Because there is nothing limiting the contraction of the copper layer 12 at a surface 16, a pulling effect, indicated by arrows 18, is created between the surface 18 of the copper layer and the area of the bond 20. The pulling effect 18 forms weak points 22 along the edges of the bond. Ceramic is weak in tension and, if pulled on, will break. Thus, over time the pulling effect 18 can cause conoidal fractures 24, i.e., cracks or craters in the ceramic layer 26.
The ceramic layer 26 is often used as an electrical insulator between the copper layer 12 and another conductive layer 28. The thickness T of the ceramic layer is one factor that determines the breakdown voltage level of the ceramic layer. Cracks or conoidal fractures in the ceramic provide shorter breakdown paths between the copper layer 12 and the conductive layer 28 which can result in an insufficiently low breakdown voltage. Moreover, even if initial cracks do not reduce the breakdown voltage to an insufficient level, "crack propagation" can lead to more cracks and larger cracks over time that may indeed result in an insufficiently low breakdown voltage.
Another difficulty with directly bonding a layer of copper to a layer of ceramic is the formation of tents between the two layers, which is often referred to as the "tenting effect." In general, tenting effect is increased as the thickness of the layer of copper is decreased. There are varying theories in the industry as to how tents are formed.
Referring to FIGS. 2 and 3, a tent 30 is an elevated area between the two layers in which eutectic bond is not formed. Tents cause many difficulties in the subsequent photo-imaging process steps. For instance, if a liquid photo-imaging layer 32 (FIG. 2) is applied to the surface 16 of copper layer 12, then the liquid may roll off the tent, indicated by arrows 34, leaving a portion of the surface 16 uncovered. If a film photo-imaging layer 36 (FIG. 3) is applied to surface 16, then the film may not lay flat against surface 16 and may leave gaps 38 between the surface 16 and the film 36. Similarly, a photo-imaging tool 40 (i.e., a negative) may also not lay flat.
Also, because the areas of the copper/ceramic sheet which contain tents represent defects, an additional manufacturing step is required to detect and remove them.